1. Field of the Invention
The present invention relates to an EBG (Electromagnetic Band Gap) structure which prevents the propagation of electromagnetic waves in a specific frequency band. More specifically, the present invention provides a member and an apparatus which reduce unnecessary radiation noise emitted from a wireless communication apparatus without decreasing the antenna radiation gain of the wireless communication apparatus by using an EBG structure member.
2. Description of the Related Art
Recently, studies have been made on an EBG technique for preventing the propagation of electromagnetic waves in a specific frequency band.
As a measure against unnecessary radiation without using any conventional EBG technique, there is available a technique of bonding a radio wave absorber that absorbs electromagnetic waves to an electrical component or transmission path which emits unnecessary electromagnetic waves in order not to emit the unnecessary electromagnetic waves outside an apparatus. If a high-frequency circuit portion of an apparatus is high-frequency coupled to a peripheral conductive member, the mechanical size of a member is resonant with a specific frequency. For this reason, in order to suppress the resonance, for example, high-frequency conduction is provided with respect to another conductive member, thereby suppressing resonance at a specific frequency.
A conventional technique of suppressing unnecessary radiation components emitted from an apparatus will be described with reference to FIGS. 9 and 10.
FIG. 9 is a perspective view schematically showing the internal structure of a wireless communication apparatus. Reference numeral 600 denotes a wireless communication apparatus; 601, a dielectric substrate housed in the wireless communication apparatus 600; 602, an antenna (antenna element or antenna pattern) mounted on the dielectric substrate 601; 603, a GND portion of the dielectric substrate 601; 604, a high-frequency unit (a module including an oscillator and an amplifier) for transmitting/receiving radio signals; 605, a conductive attachment plate for attaching/fixing the dielectric substrate 601; 606, a fixing screw; and 607, a conductive housing which is part of the constituent members of the wireless communication apparatus 600 and formed from, for example, a conductive cover.
Reference numeral 608 denotes a circle whose size represents, in a simulated manner, the intensity of radio waves emitted from the antenna 602. In addition, the lengths of the arrows in FIG. 9 indicate, in a simulated manner, the intensities of unnecessary radiation emitted from the dielectric substrate 601.
In the arrangement shown in FIG. 9, the unnecessary radiation components emitted from the wireless communication apparatus 600 exhibit large values, and the intensities of the radio waves emitted from the antenna 602 also have sufficient gains. Assume that the antenna gain is about −10 dBD.
FIG. 10 shows an arrangement obtained by providing a measure for reducing unnecessary radiation components for the arrangement in FIG. 9.
Note that reference numerals 700 to 708 denote the same members as those denoted by reference numerals 600 to 608 in FIG. 9, and hence a detailed description of them will be omitted.
In the arrangement different from that shown in FIG. 9, a conductive member 709 is disposed between the conductive attachment plate 705 and the conductive cover 707 to electrically connect the conductive attachment plate 705 to the conductive cover 707. Electrically connecting the conductive attachment plate 705 to the conductive cover 707 forms a current path along which the current components of electromagnetic waves contributing to unnecessary radiation excited by the GND portion 703 of the dielectric substrate 701 flow into the conductive cover 707 forming the wireless communication apparatus 707.
With this arrangement, the current components of electromagnetic waves contributing to the unnecessary radiation excited by the GND portion 703 partially flow into the conductive cover 707. If the current components of unnecessary radiation are harmonics of the operating fundamental frequency of the wireless communication apparatus 700 and become higher frequencies, they will be more consumed by flowing on the overall surface of the conductive cover 707. This reduces the amount of radiation from the wireless communication apparatus 700.
With the above arrangement, however, the intensity (antenna gain) of the radio waves emitted from the antenna 702 decreases to, for example, about −15 dBD.
When the antenna 702 is mounted on the dielectric substrate 701, the size of the GND portion 703 in the longitudinal direction can be about λ/4 with respect to a wavelength λ at the operating center frequency. The antenna gain changes even if the wavelength is less than λ/4 or more than λ/4.
If the conductive attachment plate 705 and the conductive cover 707 are configured to be electrically connected to each other to reduce unnecessary radiation, the GND portion 703 of the dielectric substrate 701 is high-frequency coupled to the conductive cover 707 when viewed from the antenna 702. This makes it impossible to ensure optimal mounting conditions for the antenna 702.
There is available a technique of suppressing electromagnetic waves generated from a high-frequency circuit by using an EBG structure member so as to prevent the electromagnetic waves generated from the high-frequency circuit housed in the apparatus from influencing other apparatuses.
For example, there is available a technique of suppressing currents contributing to unnecessary radiation by disposing an EBG structure member in the current path of unnecessary radiation components excited on the wall surface of a conductive material housed in the internal space of the electromagnetic waves generated from the high-frequency circuit housed in the apparatus (Japanese Patent Laid-Open No. 2011-18776).
According to Japanese Patent Laid-Open No. 2011-18776, the unnecessary radiation components of unspecified frequencies emitted from the overall conductive housing are suppressed by checking the current path of electromagnetic wave components of a specific frequency generated from the high-frequency circuit in the apparatus and disposing the EBG structure member in the current path. That is, the arrangement disclosed in Japanese Patent Laid-Open No. 2011-18776 differs from an arrangement configured to reduce unnecessary radiation in an apparatus in association with the radiation gain of an antenna like a wireless communication apparatus.
As an anti-EMI measure for a wireless substrate mounted in a wireless communication apparatus, there is available a method of reducing unnecessary radiation components emitted from a high-frequency circuit and a circuit substrate GND mounted on a wireless substrate by electrically connecting the wireless substrate GND to a conductive member of the housing. However, since the circuit substrate GND operating as the GND of the antenna element and the conductive member, which are simultaneously mounted on the wireless substrate, are electrically connected to each other, the antenna element is high-frequency coupled to the conductive member, resulting in a decrease in radiation gain.
In Japanese Patent Laid-Open No. 2011-18776 to which an EBG structure is applied, unnecessary radiation is reduced by suppressing a resonant state in the housing. However, this technique cannot be applied to an anti-unnecessary-radiation measure associated with the antenna gain of a wireless communication apparatus including an antenna.